Ink jet printers, of both the drop-on-demand and continuous jet type, have lately enjoyed popularity in several applications, because of their good quality and relatively low hardware cost. Within the drop-on-demand category, several printing technologies have been employed including "bubble jet", in which the pressure from an expanding gas bubble in a liquid chamber causes a drop to be ejected from an orifice; and "piezo", in which the pressure pulse used for drop ejection is created by a piezoelectric actuator. However, these prior printing technologies have certain disadvantages. First, there has been no means in the prior methods by which the lightness, or optical density, of the deposited ink layer could be continuously adjusted. This then necessitates the use of digital halftoning methods, in order to render images with continuous tones, or shades.
These methods require the use of small ink droplets, high printer resolution, and complex halftoning algorithms in order to achieve images of high quality. Second, since the rate at which the printed area can be covered with ink droplets decreases as the square of the resolution (given a fixed number of nozzles and firing rate), and since high image quality requires high resolution, there has been a problem in the prior methods with slow printing speed, especially for higher quality images. Third, the amount of colorant which is carried by a liquid drop to a receiver has been limited in the prior methods by the solubility of the colorant in the liquid carrier, resulting in excessive liquid deposition, curl, long drying time, and other problems.